The Minnesota Pollution Control Agency has fined Northshore Mining Co. in Silver Bay $242,973 for spraying 39,200 gallons of hazardous waste onto its property and improperly sending an equal amount to a nearby water treatment plant.

It is the fourth time since September 2010 that the taconite company has been fined for violating Minnesota pollution laws.

The agency found that Northshore Mining sprayed a "corrosive hazardous waste leachate" over its coal-ash landfill to control dust. An additional 38,900 gallons of the leachate were delivered to an authorized wastewater treatment plant in Duluth over the course of two days in 2011, but the quantity exceeded permitted levels.

The company failed to immediately report the violations and failed to properly monitor high pH levels in the leachate, the agency said.

Cliffs Natural Resources Inc., the Cleveland company that owns Northshore Mining, said the leachate was mistakenly taken to the treatment plant.

"We regret that this incident happened, but it has resulted in significant improvements to our landfill management practices," said Ed LaTendresse, manager for Northshore Mining. "We remain committed to Cliffs' core value of environmental stewardship."

Northshore agreed to take corrective action and donated $50,000 to the Western Lake Superior Sanitary District for renovation of a hazardous-waste collection site.

In February, the MPCA fined Northshore $240,175 for exceeding dust emission limits on about 30 occasions between November 2010 and May 2011. Very fine dust from its taconite-processing plant in Silver Bay, considered a breathing hazard under federal standards, accumulated repeatedly on boats at a nearby marina.

In 2011, Northshore was fined $26,087 for violating its water quality permit by pumping tainted water from a mine near Babbitt into surface water. And in 2010, the company was fined $11,750 for having faulty equipment that led to the release of an excess amount of air particulates.

Two years earlier, Northshore failed in a bid to convince the state to relax standards for the release of asbestos fibers into the air.